In current LTE (Long Term Evolution) systems, the set of modulation schemes for both downlink and uplink includes QPSK (Quadrature Phase-Shift Keying), 16QAM (16 Quadrature Amplitude Modulation) and 64QAM (64 Quadrature Amplitude Modulation), corresponding to two, four and six bits per modulation symbol, respectively. In LTE evolution, especially for the scenarios with high SINR (Signal to Interference and Noise Ratio), e.g. in small cell deployments with terminals close to cell site, one means to provide higher date rate with given transmission bandwidth is the use of higher order modulation (herein referring to modulation schemes that are higher than 64QAM, e.g. 256QAM, 512QAM) that allows for more information bits to be carried per modulation symbol. Higher order modulation can be utilized both in downlink and uplink to enhance the spectral efficiency. For example, in 3GPP (3rd Generation Partnership Project) Rel-12 Small Cell study item, it has been discussed to introduce a higher order modulation scheme (e.g. 256QAM) for the downlink.
In order to adopt higher order modulation schemes in LTE systems, new MCS/CQI index tables that cover higher order modulation need to be implemented and UE (User Equipment) receivers need to be upgraded to support higher order modulation. On the other hand, in some occasions such as for legacy UEs, current modulation schemes are desired. Hence, there is a need for a way to not only allow higher order modulation to be used, but also supports legacy UEs.